On this spot in 18th century (starting 1727) was a floating St. Isaak's bridge. It consisted of board layer on top of boats (later - barges). To let ships to pass it could separate at the middle. Bridge was named for St.Isaak's church on the other bank of Neva, where now St.Isaak Cathedral is standing.
They built 11 such bridges altogether (and only during the warm seasons - on winter the frozen river was crossed by ice-roads). But St.Isaak's bridge was the first, created by the order of St.Petersburg's Governor A.D. Menshikov. No wonder the bridge is close to his palace on Vasilievsky Island.
In various shapes the bridge lasted very long time, only slightly changing the location. In 1916 it burned in a fire started by a spark from the passing tugboat. So they had to open in a hurry the Palace bridge, yet unfinished.